Electron-Transfer-Chain reaction

Reactions occurring by an electron transfer chain reaction in aliphatic systems were reported independently in 1966 by the groups of Kornblum " and RusselP (equations 88, 89). These reactions involve initial transfer of an electron forming a radical anion 56b, which expels an anionic leaving group forming a neutral free radical, and this radical combines with radical 56a forming the product. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Hartl F, Rossenaar BD, Stor GJ, Stufkens DJ (1995) Role of an electron-transfer chain reaction in the unusual photochemical formation of five-coordinated anions [Mn(CO)3(-a-diimine)]- from Fac-[Mn(X)(CO)3(a-diimine)] (X=halide) at Low temperatures. Reel Trav ChimPays Bas 114(11-12) 565-570. doi 10.1002/recl. 19951141123... 

A third factor to be considered in this triangular competition (internal deactivation, back electron transfer, chemical reaction) is the possibility of an unfavored chemical reaction followed by the initiation of a long chain . 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

The electron-transfer chain (ETC) catalytic process (or, electrocatalysis) is the catalysis of a reaction triggered by electrons (through a minimal quantity of an oxidizing or reducing agent) without the occurrence of an overall change in the oxidation state of the reagent. 

Cytochrome c oxidase is a copper protein, which, in the respiratory electron-transfer chain of mitochondria and many bacteria, catalyses the reduction of molecular oxygen to water, according to the reaction ... 

Once an enzyme-catalysed reaction has occurred the product is released and its engagement with the next enzyme in the sequence is a somewhat random event. Only rarely is the product from one reaction passed directly onto the next enzyme in the sequence. In such cases, enzymes which catalyse consecutive reactions, are physically associated or aggregated with each other to form what is called a multi enzyme complex (MEC). An example of this arrangement is evident in the biosynthesis of saturated fatty acids (described in Section 6.30). Another example of an organized arrangement is one in which the individual enzyme proteins are bound to membrane, as for example with the ATP-generating mitochondrial electron transfer chain (ETC) mechanism. Intermediate substrates (or electrons in the case of the ETC) are passed directly from one immobilized protein to the next in sequence. 

Cytochrome P-450 has been characterized in four stable states [Fe, Fe " RH, Fe RH, (O2—Fe ) RH (metastable)] during its oxygenase reaction cycle. In the complete native system a flavoprotein and a redoxin (putidaredoxin) act as electron donors but also as effectors that complement the cytochrome. In the more complex microsomal system the sequence and intermediates are less well defined the electron-transfer chain contains two flavoproteins and one cytochrome, whose interactions with cytochrome P-450 are still the subject of great controversy. 

Cytochrome c and cytochrome/both are involved in the electron transfer chain from glucose metabolites to molecular oxygen in aerobic organisms. From values of the half-cell potentials of cytochrome c and of cytochrome/at 30°C, it is possible to calculate that is 0.11 V for the reaction in Equation (12.12) [4]. Hence... 

The Krebs cycle will only operate when the hydrogen atoms and electrons produced in the cycle enter the electron transfer chain, ultimately to react with oxygen that is, the two processes must take place simultaneously. A metabolic pathway is defined as a sequence of reactions that is initiated by a flux-generating step. In the cycle, citrate synthase catalyses the flux-generating reaction (Table 9.2) but there is no such reaction in the electron-transfer chain. Consequently, the cycle can be considered to be the first part of a longer pathway, which includes the electron transfer chain (Figure 9.3). 

Under aerobic conditions, the hydrogen atoms of NtUDH are oxidised within the mitochondrion pyruvate is also oxidised in the mitochondrion (Figure 9.15). However, NADH cannot be transported across the inner mitochondrial membrane, and neither can the hydrogen atoms themselves. This problem is overcome by means of a substrate shuttle. In principle, this involves a reaction between NADH and an oxidised substrate to produce a reduced product in the cytosol, followed by transport of the reduced product into the mitochondrion, where it is oxidised to produce hydrogen atoms or electrons, for entry into the electron transfer chain. Finally, the oxidised compound is transported back into the cytosol. The principle of the shuttle is shown in Figure 9.16. 

Citrate synthase There are three properties of citrate synthase that are relevant to regnlation. The prodnct of the reaction, citrate, is an allosteric inhibitor of the enzyme. The concentration of acetyl-CoA in mnscle is weU above the ATm of citrate synthase for acetyl-CoA. Conseqnently, the activity of this enzyme is flnx-generating for the cycle pins the transfer of electrons along the electron transfer chain, i.e. the process from acetyl-CoA to molecnlar oxygen can be considered as a transmission seqnence , as defined in Chapter 3. In contrast, the concentration of oxaloacetate is weU below the ATm, so that variations in its concentration can regnlate the enzyme activity and therefore, the flnx throngh the cycle. 

An important point in the regulation of these processes is that all the reactions from mitochondrial NADH, to and including cytochrome c, are near-equilibrium (Figure 9.26(a)) (Appendix 9.8) that is, there is only one reaction in the electron transfer chain that is non-equihbrium - the terminal reaction catalysed by cytochrome oxidase. There is some similarity with the process of glycolysis in which the initial reaction and the terminal reactions are the nonequilibrium reactions (Figure 9.26(b)). 

Figure 9.26 (a) Near-equUibiium and non-equilibrium reactions in the electron transfer chain. The electron transfer chain is considered to be the Latter part of the physiological Krebs cycle (see above). The non-equilibrium processes are the Krebs cycle and the terminal reaction cytochrome oxidase. All other reactions are near-equilibrium, including the ATP-generating reactions. These are not shown in the figure, (b) The similarity of electron transfer chain and glycolysis in the position of near-equilibrium/non-equilibrium reactions, in the two pathways. In both cases, non-equilibrium reactions are at the beginning and at the end of the processes (see Chapters 2 and 3 for description of these terms and the means by which such reactions can be identified). 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

A principle in metabolic regulation that allows one to identify the inhibited step within a metabolic pathway as that reaction for which the concentrations of reactants and products rise and fall, respectively, from their steady-state values when an inhibitor is introduced. In the context of the electron transfer chain, the crossover-point refers to that reaction step demarking the transition from more reduced to more oxidized respiratory enzymes. 

Kim and Bunnett in 1970 made unexpected observations of reactions occurring by an electron transfer chain mechanism in aromatic systems. " The selective formation of 60 showed that benzyne intermediates were not formed, and the mechanism of equations (90) and (91) analogous to that found in aliphatic systems (equations 88, 89) was proposed. This process differs in being a chain reaction and not a process in which the product forming step is not a radical-radical combination. 

In connection with the oxirane ring opening, the isomerization of oxirane 6 to ketone 9 in an MeCN—LiClO —(Pt) and a CH Clj—Et NClO —(Pt) system has been reported and was explained on the basis of an electron-transfer chain mechanism via 7 and 8 However, 6 is actually converted to 9 under the preelectrolysis conditions so that the transformation may be explained in terms of an EGA catalyzed reaction... 

In the overall reaction catalyzed by the mitochondrial respiratory chain, electrons move from NADH, succinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes, and finally to 02. A look at the methods used to determine the sequence in which the carriers act is instructive, as the same general approaches have been used to study other electron-transfer chains, such as those of chloroplasts. 

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